In many broadcasting applications, it is common for audio signals from various input devices to be fed to a central console from which they may be processed and/or broadcast to other locations. Typically the central console includes some type of mixer device by which the input audio signals may be routed to external devices, such as VTRs, or to broadcast devices which may feed a live signal.
Such an arrangement usually includes some type of means of controlling whether an input device such as a microphone is currently active. For example, the central console may include a mute switch which interrupts the signal path of the microphone. Further, the console may include an adjustable fader for adjusting the level of the audio channel which may be set at full attenuation of the signal.
In such applications, it is desirable to indicate to the user that an input channel is active, that is, currently "on air" or recorded. Such a tally helps prevent unwanted remarks, comments or noises from inadvertently being broadcast. For example, a sign stating "ON AIR" may be illuminated or some other prominent display may be used to alert the user that a microphone is active.
In the past, such indications have been provided by use of a simple "on air" light activated from a single button or switch. Alternatively, a tally of the fader control has been used. Such prior approaches are limited in several respects. For example, by monitoring only a single point at which the signal is interrupted, false on air signals may be sent in the event of failure of the monitoring circuitry. Moreover, such prior techniques often couple the console with the lamp which provides the tally. This introduces the risk of drawing power into the console which may interfere with the signals that are being broadcast or processed with the console.
Accordingly, there is a need to provide a method and apparatus by which proper signalization, that is, an output signal indicative of a true on air event, is accomplished in a reliable manner without the above-described limitations of the prior art.